The present invention relates to coated optical fibers and cables, and more particularly to coated optical fibers and cables of novel coating design which are particularly well suited for blown optical fiber applications.
The terms "blow," "blown" and "blowable" as currently used in optical fiber design and optical telecommunications technology refer to optical fibers and optical fiber cables which may be installed in existing ductwork by the effect of viscous fluid drag. The fibers or cables are sufficiently small and/or light, and the ducts sufficiently close in size to the fibers or cables, that significant forces tending to draw the fibers or cables through the ducts can be generated by forcing a gas such as air through the ducts and past the fibers or cables in the direction in which they are to be drawn.
U.S. Pat. No. 4,796,970 describes optical fiber transmission media utilizing blown fibers and methods for using the same. As pointed out in that patent, optical fiber cables comprising glass optical fibers have generally been installed by methods similar to those for metal cable installation, i.e., by pulling them through the ducts with pulling cables.
However, unlike conventional wire cables, optical cables contain very thin glass fibers which are easily damaged by tensile stress. To protect these fibers, optical cables have traditionally included bulky metal or other strength members which serve no purpose other than to impart tensile strength sufficient for cable installation.
Due to the difficulty of installing optical fiber cables in existing ductwork, it is often the case that more fibers or cables than actually needed are installed. This is obviously uneconomical, and it carries the further risk that the extra installed fiber capacity will become obsolete due to advancing fiber technology before the installation investment is recovered.
In the blow-in method of optical fiber installation described in the aforementioned patent, the fibers are propelled through small conduits in multi-channel ducting by viscous fluid drag generated by an injected air, nitrogen, or other suitable gas stream. This technique is intended to apply distributed force over the entire length of the surface of the fibers, avoiding localized stresses and greatly simplifying fiber installation in pre-installed ducts.
To be suitable for blow installation, the fibers or fiber groups disclosed in the patent are provided with a loosely-fitting polymer sheath composed of polyethylene or polypropylene. The sheath may optionally be textured to increase the amount of viscous fluid drag.
Later patents describe a number of improvements in blowable optical transmission media, or in methods or equipment for media installation. Published British Patent Application 2,156,837, for example, discloses the use of adherence-reducing substances in the wall portion of conduits intended for blown fiber installation. These substances increase the length of conduit through which an optical fiber can successfully be installed.
A further coating modification, intended to improve blowability while reducing low temperature fiber buckling within the blowable sheathing, is disclosed in published European patent application EP 0157610. In that design, multiple fibers are enclosed in a dual sheath comprising an inner sheath of relatively high density and elastic modulus (e.g., polypropylene) and an outer sheath of low density and modulus. The outer sheath is formed of a cellular polymer such as foamed polyethylene. The inner, high-modulus sheath is formulated and configured to resist the longitudinal compressive stress generated by shrinkage of the outer sheath at low temperatures.
Published British patent application GB 2,206,420 describes still another design for a blowable optical fiber cable. That cable comprises multiple coated optical fibers fused into a readily separable array, generally without tight cable jacketing. The coated fibers are individually blowable and readily separated from the cable at desired branching or termination points.
The desirable blow-in characteristics of the cable and fibers are imparted by the outermost coating layer on the fibers which consists of a low-density foamed polymer such as foamed polyethylene. The low-density layer is provided as an overlayer on an underlying coating material, which is generally a conventional uv-cured acrylate protective coating. Foamed acrylates statedly comprise an alternative outer sheath material for imparting good blow-in behavior.
Published British patent application 2,206,220 discloses a design for a blowable optical fiber cable wherein multiple optical fibers (with conventional coatings) are encased together in a soft, low elastic modulus inner sheath, rather than in a high modulus sheath as in EP 0157610. The design further includes an outer sheath of a low-density foamed polymer for blowability and preferably an intermediate layer of hard high-modulus material for fiber protection.
Many of the prior art cable structures designed for blow installation are intended to be of substantial stiffness. This is because a preferred method of installation for multi-fiber cables involves initially pushing the cable into the duct, with viscous fluid drag being used as an assist to complete the installation. This technique cannot be used where single fiber installations or light flexible cable configurations are required.
A further disadvantage of some of the prior art designs involving foamed or expanded polyolefin outer coatings is that extrusion or solvent blowing methods must be used to form the foamed coatings. Extrusion techniques utilize viscous materials and require additional equipment investment, with careful process control to avoid optical attenuation in the fiber due to coating stress. Solvent blowing methods frequently require the use of expensive pollution abatement or exhaust systems in fiber manufacture.
Finally, previous blowable fiber and cable designs do not adequately protect the optical fibers from excessive signal loss at low temperatures. Foamed polymer coatings of the kinds used for blown fibers are inherently non-homogeneous, the coatings generally consisting of assemblages of gas-filled voids encased in continuous polymer matrices which are themselves not uniformly dense. Hence foamed polymer coatings, and especially thick foamed coatings, can substantially increase signal loss due to fiber microbending during fiber operation at low temperatures, i.e., in the vicinity of about -60.degree. C.
It is therefore a principal object of the present invention to provide a blowable optical fiber having excellent blow-in properties in combination with very low microbending loss at low temperatures.
It is a further object of the invention to provide single optical fibers and multi-fiber optical cables, and a coating design for the same, which are particularly suitable for blow installation.
It is a further object of the invention to provide a method for manufacturing a blowable optical fiber which utilizes liquid draw coating rather than extrusion to provide an outer coating for the fiber.
It is a further object of the invention to provide a coating process for making blowable optical fibers and coatings which does not rely on solvent blowing methods and associated pollution abatement or exhaust systems.
It is a further object of the invention to provide a method for making blowable optical fiber and cable which offers better dimensional control over the applied low-density foamed polymer coating than prior art methods.
It is a further object of the invention to provide a coating system and blowable optical fibers or cables comprising the same which offer flame retardant properties.
It is a further object of the invention to provide a coating system and blowable optical fibers or cables comprising the same which permit the convenient color coding of the fibers or cables by the inclusion of identifying colorants in the coatings.
Other objects and advantages of the invention will become apparent from the following description.